New Brain Cells, Stress, And Learned Behavior

A new study by UT Southwestern scientists (Lagace, Donovan, DeCarolis, Farnbauch, Malhotra, Berton, Nestler, Krishnan and Eisch) sheds some light on the connection between stress and neurogenesis.

Eisch and her colleagues performed two experiments related to stress.

1. They exposed mice to a socially stressful experience — confrontation with a more aggressive mouse (the mouse equivalent of a carjacking), then measured the immediate and long term impact on the generation of new brain cells.

2. They irradiated mice to eliminate neurogenesis before exposing the irradiated mice to the same kind of stressful situation.

The scientists made two important findings:

In the first experiment, the stressful situation reduced neurogenesis temporarily (for a few days), and left the mice more likely to be fearful in similar situations.

In the second experiment the irradiated mice showed less fear when exposed to similar stressful situations.

These findings indicate that neurogenesis is key to forming stress memories. This can be a healthy response, educating us on avoidance. (Common sense.) But in cases of inappropriate or chronic stress response, neurogenesis may be overactive.

Under normal circumstances the amygdala would be involved in the direct response to the stressful event as well as any reinforcement or diminishment of this response over time. The hippocampus, I guess, would play an accompanying role in the formation and modulation. And whereas the amygdala forms the instinctive emotional memory, the hippocampus would store more of the conscious awareness of the stressful event and trigger.

One would presume that neurogenesis provides new cells that allow the new memories to take hold. Restricting neurogenesis stops the fear from taking hold as a memory or learned response.

The difference between the amydgala and hippocampus would seem to be in their roles in the instinctive / emotional versus the more processed or conscious awareness. In PTSD or other conditions the fear response has been so extreme that it forms a persistent, chronic, and problematic condition. I guess that’s where blocking neurogenesis at the appropriate time and to the appropriate degree can be quite beneficial if this can be harnessed correctly.

After the fact, the response needs to be reconditioned. I know that people are working on the theory that reconditioning can be aided by boosting neurogenesis. That’s an intriguing theory and provides reason for optimism. Since chronic stress in itself inhibits neurogenesis and therefore limits the possibility that the stress can be reconditioned, then boosting neurogenesis might shortcut this cycle and permit the stress to be overcome.